1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus and a process cartridge for use in electrostatic copiers and laser printers, etc.
2. Discussion of the Background
Organic photoreceptors are mostly used in image forming apparatus using electrophotographic process because of having many advantages of being more low-cost, less toxicity, easier-to-coat, etc. than inorganic photoreceptors having been vacuum-evaporated with selenium. Particularly, the organic photoreceptors wherein a charge generation layer and a charge transport layer are layered on an electroconductive substrate are mostly used.
However, the organic photoreceptor is liable to be abraded when repeatedly used, which causes deterioration of the charge potential and photosensitivity thereof and damages of the surface thereof, resulting in background fouling and deterioration of image density of the resultant images. Therefore, improvement of the abrasion resistance and scratch resistance of the organic photoreceptor is desired.
Further, the improvement of the abrasion resistance of the photoreceptor is essential because of the recent higher-speed of electrophotographic apparatus, smaller diameter of the photoreceptor with downsizing of the apparatus, and further full-colorization and free maintenance thereof. Therefore, it is most essential for the organic electrophotographic photoreceptor to produce high-quality images and have high durability.
Various materials are added into the charge transport layer to improve property of the surface of the photoreceptor and durability thereof.
Japanese Laid-Open Patent Publications Nos. 10-198053 and 10-301303 disclose that an electrophotographic photoreceptor including an epoxy compound and a hindered amine or phenol compound having a specific constitution in its photosensitive layer has high durability of not changing electrical properties such as charge potential and residual potential, and not changing sensitivity and image quality without swollen letters and blurred images.
Japanese Laid-Open Patent Publication No. 10-198053 and 10-301303 disclose that an electrophotographic photoreceptor including a polytetrafluoroethylene powder as a lubricant and 4,4-diphenyl-1,1-di(P-di-lower-alkyl-substituted amino phenyl)-butadiene having a formula (C6H5)2(C6H5P—NR2)2 in a surface layer of its photosensitive layer as a charge transport material has durability against abrasion due to friction and scratch, produces high-quality images without blur and has good cleanability without toner adherence to the surface thereof.
Japanese Laid-Open Patent Publication No. 2002-139859 discloses that an electrophotographic photoreceptor including an electroconductive substrate, a photosensitive layer thereon and a protective layer including a filler thereon, wherein the photosensitive layer includes an organic sulfuric compound and the protective layer includes both a hindered amine constitution and a hindered phenol constitution, does not increase the residual potential, does not produce poor images such as blurred images, and stably produce high-quality images for long periods.
However, any of these conventional technologies are insufficient in terms of simultaneous pursuit of higher-quality image and higher durability.
For example, the polytetrafluoroethylene powder disclosed in Japanese Laid-Open Patent Publication No. 10-198053 is a polymer having a low surface energy, and is insoluble in a solvent and has poor dispersibility, resulting in difficulty in preparing a photoreceptor having a smooth surface. Although the dispersibility can be improved with a dispersant, light tends to scatter because the resultant coated layer has a small flexibility, resulting in deterioration of latent images and production of blurred images.
In addition, the photoreceptor disclosed in Japanese Laid-Open Patent Publication No. 2002-139859 has good durability, but does not produce such high-quality images as the photoreceptor of the present invention does.
Further, as conventional technologies to improve the abrasion resistance, (1) Japanese Laid-Open Patent Publication No. 56-48637 discloses a hardening binder used in a crosslinked charge transport layer, (2) Japanese Laid-Open Patent Publication No. 64-1728 discloses a charge transport polymer material, and (3) Japanese Laid-Open Patent Publication No. 4-281461 discloses an inorganic filler dispersed in a crosslinked charge transport layer. A photoreceptor using the (1) hardening binder tends to increase the residual potential and produce images having low image density because of poor compatibility of the hardening binder with a charge transport material and impurities such as a polymerization initiator and an unreacted residue. A photoreceptor using the (2) charge transport polymer material has good abrasion resistance, but does not have satisfactory durability. In addition, the charge transport polymer material is difficult to polymerize and purify, and a coating liquid including the charge transport polymer material has high viscosity. A photoreceptor using the (3) inorganic filler has a higher abrasion resistance than a conventional photoreceptor wherein a low-molecular-weight charge transport material is dispersed in an inactive polymer, but tends to increase the residual potential and produce images having low image density because of a charge trap present on the surface of the inorganic filler. In addition, when the inorganic filler and a binder resin form large concavities and convexities on the surface of the photoreceptor, the cleanability hereof deteriorates, resulting in toner filming and distorted images. These technologies of (1), (2) and (3) do not fully satisfy an overall durability including electrical durability, mechanical durability and image durability required for an organic photoreceptor.
Further, in order to improve the abrasion resistance and the scratch resistance of the photoreceptor using the (1) hardening binder, Japanese Patent No. 3262488 discloses a photoreceptor including a hardened polyfunctional acrylate monomer. However, although disclosing that a protection layer thereof includes the hardened polyfunctional acrylate monomer, Japanese Patent No. 3262488 only discloses that a charge transport material may be included therein and does not disclose specific examples thereof. In addition, when a low-molecular-weight charge transport material is simply included in a surface layer, the low-molecular-weight charge transport material is not soluble with the hardened polyfunctional acrylate monomer and the low-molecular-weight charge transport material separates out and becomes cloud in the surface layer, and which causes deterioration of mechanical strength of the resultant photoreceptor.
Further, since the hardened polyfunctional acrylate monomer is reacted in a surface layer including a polymer binder, a three-dimensional network is not fully developed therein and the crosslinking bond density is thin, and therefore the photoreceptor does not have noticeable abrasion resistance.
As an abrasion resistance technology of a photosensitive layer in place of these technologies, Japanese Patent No. 3194392 discloses a method of forming a charge transport layer using a coating liquid formed from a monomer having a carbon-carbon double bond, a charge transport material having a carbon-carbon double bond and a binder resin. The binder resin improves adhesiveness between a charge generation layer and a hardening charge transport layer, and is considered to lessen an internal stress therein when hardening. In addition, the binder resin has a carbon-carbon double bond and is broadly classified to a binder resin having a carbon-carbon double bond and reactivity with the charge transport material and a binder resin not having a carbon-carbon double bond and reactivity therewith. The photoreceptor has both good abrasion resistance and electrical property, but when the binder resin not having a reactivity with a charge transport material is used, the binder resin has poor compatibility with a hardened material produced by a reaction between the monomer and charge transport material and the crosslinked transport layer has a layer separation therein, and which occasionally causes damages, and adherence of an external additive in a toner and a paper powder on the surface of the photoreceptor. As mentioned, above, a three-dimensional network is not fully developed therein and the crosslinking bond density is thin, and therefore the photoreceptor does not have noticeable abrasion resistance. In addition, the monomer is bifunctional, which is unsatisfactory in terms of the abrasion resistance. Even when the binder resin having a reactivity with a charge transport material, although having more molecular weight, the hardened material has less intermolecular crosslinking bond. Therefore, the resultant photoreceptor does not have a sufficient crosslinking density, and the electrical and property and the abrasion resistance thereof are not satisfactory.
Japanese Laid-Open Patent Publication No. 2000-66425 discloses a photosensitive layer including a hardened positive hole transport compound having two or more chain polymeric functional groups in the same molecule. Although having a high hardness because of its high crosslinking bond density, the photosensitive layer includes a bulky positive hole transport material having two or more chain polymeric functional groups, and a distortion appears in the hardened compound and an internal stress increases to cause a crack on the crosslinked surface layer and a peeling thereof.
Further, Japanese Laid-Open Patent Publication No. 2001-166501 discloses a photosensitive layer including a positive hole transport polymer compound having two or more chain polymeric functional groups in the same molecule, and having a thickness of from 5 to 15 μm. The thickness is limited to stabilize the electrostatic properties of the photoreceptor, such as charge potential and residual potential thereof in addition to the abrasion resistance and scratch resistance thereof. The thickness does not refer to image quality and a relationship thereof with a writing light source is not referred at all.
In order to improve the abrasion resistance, a photosensitive layer or a surface protective layer including an organic silicon binder resin is practically used. However, the organic silicon binder resin has a high hygroscopicity, resulting in deterioration of image quality, such as blurred images due to toner filming and distorted images.
The reason why a photoreceptor producing high-quality images and having high durability is difficult to prepare is considered as follows.
As mentioned above, many trials have been made to realize high abrasion resistance, and the organic photoreceptor has been noticeably improved in its abrasion resistance. However, at the same time, abnormal images such as blurred images have noticeably been produced. This is because a charged product becomes difficult to remove as the abrasion resistance of the photoreceptor improves.
Namely, since a conventional photoreceptor has a low abrasion resistance, the charged product is removed with abrasion even when accumulated on the surface thereof. The blurred image is produced by a blurred electrostatic latent image due to a horizontal movement of a charge because of lowering of the surface resistivity of the photoreceptor. The surface resistivity lowers because ion species (hereinafter referred to as a charged product) produced with ozone and NOx generated when the photoreceptor is charged and atmospheric moisture are accumulated thereon. In addition, as the abrasion resistance is improved, scratches on the surface of the photoreceptor are difficult to remove and the charged products accumulated therein are more difficult to remove.
Further, besides the charged products, the time and number for the surface of the photoreceptor to be exposed stresses such as a charge noticeably increase as the abrasion resistance thereof improves. Therefore, the surface of the photoreceptor occasionally changes in quality due to ionization and bond breaking because of the stress. Although this may not cause blurred images, but the resistivity locally lowers, resulting in expansion of a dot and deterioration of image density gradation. It is not clear whether a moisture accelerates such reactions or is a medium of lowering the resistivity, however, the phenomena noticeably occurs in an environment of high-temperature and humidity. Therefore, it is essential that the photoreceptor has high abrasion resistance and produces high-quality images as well.
As mentioned above, the blurred images are produced by ozone and NOx generated when the photoreceptor is charged, and adherence and accumulation of the charged products thereon. Therefore, the blurred images can be prevented when the ozone, NOx and charged products increase.
Charges for use in the electrophotographic image forming apparatus are broadly classified to non-contact chargers like corona chargers charging the photoreceptor without contacting the photoreceptor, such as a corotron and a scorotron; and contact chargers charging the photoreceptor contacting the photoreceptor, such as a charging roller and a charging brush.
Corona chargers are simple, but generate much ozone and NOx, adversely affects the electrostatic and image properties of the photoreceptor as well as the environment. In addition, the corona chargers are disadvantageous for downsizing the apparatus. Particularly, when installed in an electrophotographic tandem image forming apparatus including plural photoreceptors and chargers, the ozone and NOx are generated more.
On the other hand, the contact chargers such as a charging roller and a charging brush can charge the photoreceptor to have a specified potential with a lower applied voltage, and therefore the ozone and NOx largely decrease. Therefore, the contact chargers have an advantage against the blurred images, and can uniformly charge the photoreceptor. However, contacting the photoreceptor, the charging member thereof is contaminated with a toner and foreign particles adhering thereto, resulting in deterioration of the chargeability of the photoreceptor; the photoreceptor is contaminated with the toner and foreign particles adhering a gain thereto; the abrasion or uneven abrasion of the photoreceptor is accelerated with the contaminated charging member; the foreign particles removability deteriorates because the charging member is pressed against the surface of the photoreceptor; and abnormal images are produced due to a trace of the charging roller on the photoreceptor or a deformation thereof.
A method of locating a charging member close to a photoreceptor can decrease the ozone and NOx as the contact chargers and can reduce the above-mentioned problems of the contact chargers as well because of not directly contacting the photoreceptor, which seems an effective method for a photoreceptor to have higher durability and produce higher quality images. In addition, the charging member in this method is not easily contaminated and easy to remove the foreign particles even when adhered thereto, and therefore the charging member and the photoreceptor have higher durability.
However, the charging member and the photoreceptor need to precisely be located each other, and the charge stability of the photoreceptor tend to rather deteriorate. An AC voltage overlapped with a DC voltage is applied to the charging member to improve the charge stability, which increases the charged products, resulting in production of blurred images. Further, a charging member located close to a conventional photoreceptor wherein a specific outermost layer is not formed on a photosensitive layer, more causes discharge breakdown of the surface of the photoreceptor. Therefore, even when a charging member is located close to a photoreceptor, the photoreceptor does not have higher durability as expected.
In order to realize a downsized and energy-saving electrophotographic image forming apparatus having high durability which does not need a replacement of a photoreceptor and a dehumidifier and producing high-quality images, the abrasion resistance of the photoreceptor needs to be improved and production of blurred images needs to be prevented. In order to prevent production of blurred images, it is essential to prevent generation of ozone and NOx and contamination of a charging member. The charging member located close to a photo receptor seems to be effectively used, however, as mentioned above, the blurred images and the discharge breakdown of the photoreceptor rather increase, and is not effective for producing high-quality images.
Because of these reasons, a need exists for a power-saving and downsized electrophotographic image forming apparatus having a long life and stably producing quality images for long periods.